Antimicrobial Resistance Arising from HGT between Food and Animals
Antibiotics are commonly used as a means by which the field of agriculture and livestock treat and prevent dieseases and also to promote the growth of animals at sub-therapeutic doses. Because such a huge number of antibiotics are used so commonly, this results in the development of antimicrobial resistance (AMR), which is mostly found in the intestinal microbiota of food animals. For this reason, animal manure contains the highest amount of AMR in a single reservoir. Most of this AMR is carried by commensal bacteria (bacteria who benefit from colonization while not affecting others), AMR genes can be transferred to pathogens of humans and animals through horizontal gene transfer (HGT), giving rise to the characteristic that animal manure is harmful to human health. Use of Antibiotics in Food Animal Industry and Development of Antimicrobial Resistance (1) During commerical food animal production (e.g. cattle), large quantities of antimicrobials are used to treat and prevent disease and promote growth, as stated above. Those antibiotics primarily used for the treatment and prevention of disease include penicillins, macrolides, polypeptides, streptogramins, and tetracyclines. Most bacteria carried by individual animals are located within the intes tinal tract (about 10^11 bacteria/g fecal content) and in mammals, bacteria make up about 50% of the fecal species. When antimicrobials are consistently fed to food animals over extended periods of time, intestinal bacteria are under constant selective pressure to develop resistance to commonly administered antibiotics, like those listed above. Because of this, AMR develops primarily in the intestinal tract and feces becomes the single largest reservoir of AMR arising from food animal production. Figure 1 illustrates how resistance genes can be transferred into bacteria that is pathogenic to animals and humans through HGT and vertical gene transfer (VGT). Multi-drug resistance develops from clustering of multiple AMR genes together, mostly on mobile genetic elements. Because of this, selection by one antibiotic drug can co-select multi-drug resistance (MDR). An example of this can be seen in beef cattle who are fed tylosin (a macrolide) which is the root of tetracycline resistance. Also important, multiple AMR genes are physically located on the same mobile genetic elements and therefore can cause MDR which can be transferred to other bacteria through HGT. Impact of AMR in livestock manure on the development of AMR in the environment Animal manure from food animals is generally applied to land directly or after initial treatment/storage in on-farm animal facilities, thus introducing antimicrobial resistant bacteria to soil which are then disseminated to both soil and water environments. The surivival of manure bacteria after land application varies dramatically. An example of this can be seen by human pathogens like Salmonella, Campylobacter, Yersinia enterocolitica and E. coli O157:H7 which have survivals in water, soil, and manure slurry varying from one day to over a year. Because of the potentially long surivival times of human pathogens from manure that has been disseminated from food animal manure, AMR is becoming an increasing human health concern because these AMR commensal bacteria function as a huge resistance reservoir and can spread AMR to the environment and humans. Mobile Genetic Elements and HGT of resistance genes AMR genes between soil bacteria and bacteria from animal origins are transferred via broad host-range plasmids. These mobile genetic elements have been linked to the emergence of MDR. Conjugative plasmids, transposons, integrons, phages, and insertion elements have all been implicated in HGT. References 1. http://www.intechopen.com/download/get/type/pdfs/id/34705 Section heading Write the second section of your page here.